This invention relates to a fitting which is adapted to receive the end of plastic tubing and to hold the tubing firmly in place despite forces which might be encountered in use tending to separate the tubing from the fitting. The fitting, in turn, is adapted to be inserted into the locked in place in a female connector. The connector is adapted to form a sealing contact with the end of the plastic tubing. One of the problems which has arisen in the past is that any given fitting operated successfully with plastic tubing only within a limited degree of hardness. Tubing with of a hardness outside of such limited value was too easily pulled out of locked engagement with the fitting. Therefore, one of the principal objects of this invention is to devise a fitting which will receive and firmly retain tubing throughout a relatively broad range of hardness.